1. Field of the Invention
The present invention relates to a head-up display device mounted on a vehicle and a display unit including the head-up display device, and in particular, relates to a technology for warning against excessive speed of the vehicle.
2. Description of the Related Art
Head-up display devices mounted on vehicles are typically configured such that display light emitted from a projection unit is projected onto an optical member (half mirror) called a combiner; and light reflected off a surface of the combiner is reflected in a viewing direction of the driver and guided to a predetermined eye point as a viewing position of the driver (see, for example, Japanese Patent Application Laid-open No. 2006-11122). The combiner is disposed at a position in front of the driver's seat, for example, above the instrument panel, that is easily visually recognizable by the driver. A front windshield, instead of the combiner, may be used as the surface for reflecting the display light.
The head-up display device forms a display image as a virtual image at a position anterior to the combiner in the vehicle forward direction (that is, posterior in the viewing direction). This offers an advantage that the driver requires only small focus adjustment for visually recognizing the display image. In addition, the head-up display device allows the driver to visually recognize the display image anterior to the combiner in the vehicle forward direction superimposed on a view anterior to the vehicle that is visually recognized through the combiner. The head-up display device can thus present information in a novel mode different from display using an ordinary display panel.
Such a head-up display device can be used for, for example, displaying the vehicle traveling speed. The head-up display device is therefore suitable for displaying highly important information such as vehicle speed, because the driver can visually recognize the speed display with his/her head up and without involving any considerable movement of the sight line or focus adjustment.
Speed limits are set for various roads on which vehicles travel based on the Road Traffic Law and the like. For example, the maximum speed for roads with no speed limit sign and marking is set to 60 km/h for open roads and 100 km/h for highways. Furthermore, the speed limits are determined in accordance with the respective road conditions, and the speed limits therefor are explicitly informed with signs and markings on the roads.
The driver of a vehicle will normally drive the vehicle at a speed within the speed limit range so as not to violate the provisions of the Road Traffic Law and the like. That is to say, the driver will grasp the speed limit set for the road on which the vehicle is traveling currently with signs and markings on the road, read the current traveling speed of the own vehicle from the display on a speed meter, and drive the vehicle while confirming that the vehicle speed does not exceed the speed limit.
Even when the driver tries to follow the provisions of the Road Traffic Law and the like, the driver can mistakenly recognize the speed limit set for the current road due to oversight of the sign or marking on the road, for example. The driver can mistakenly recognize the speed display when he/she visually recognizes the display on the speed meter.
The present invention has been made in view of the above-mentioned circumferences, and an object thereof is to provide a head-up display device that allows a driver to easily grasp a relation between the current traveling speed of the vehicle and the speed limit set for a road on which the vehicle is traveling currently, and a display unit including the head-up display device.